Oh ouch, I agree with Shelle that you may want to reconsider the wedding party.  Even an amicable divorce is going to make both of them standing up there awkward.

She may be the bigger person, but HE may not.

Divorce is hard, she is going to be depressed and probably talk about it a lot, she wants/needs to get it all out and since you are her sister it's natural she would turn to  you.

I can imagine she is jealous, you and your FH apparently have something she doesn't with her husband.  Plus she coudl be remembering their wedding day as you plan for yours.

It sucks that she is a Debbie Downer, but can you blame her? 

Try to be there for her and I would suggest toning down discussing the wedding. I know she is your MOH but unless the wedding is tomorrow you don't have to constantly talk about it.  Do other things together that can distract her from marriage, weddings and her husband.

This is a hard time for your sister.  Try your best to be there, but don't let it color your own upcoming marriage either.

Please be as compassionate and understanding as you can.  I'm sure that your wedding plans are as painful to her as a baby shower would be for someone who is having fertility issues.

As much as you are having fun looking forward to planning and enjoying your day and the wedded bliss to come, your sister's life is upside down -- divorce affects every piece of your life -- job, home, money, children, health, etc.  So cut her as much slack as possible.

And, yes, for goodness sake, her soon-to-be-ex-husband can't be the Best Man.  It would be so painful for your sister, not to mention the in appropriateness of a we'll-be-divorced-any-minute-now couple standing up there with you as you take your own vows.

Best wishes.